It is reported that carbon-based plants present in the Earth reach 10 times of the world oil consumption and 5 times of all kinds of energy consumption. Biomass products used in the world are sequentially rice straw, wheat straw, corn stover, and bagasse, and the total amount of the biomass products is 1,549 trillion kg. Recently, to stably produce plant bio energy attracting attention as the next generation environmentally friendly energy source capable of replacing fossil fuel, attention to development of biomass plants having high productivity is increasing. Raw materials of alternative energy bioethanol which now become the biggest issue are roughly classified into saccharides (sugar cane, sugar beet, etc.), starches (corn, potato, sweet potato, etc.), and lignocelluloses (lumber from thinning, waste wood, rice straw, etc.). All of commercialized bioethanol-producing techniques use food resources such as saccharide-based and starch-based biomasses as raw materials, and have a very close relationship with food supply for humans. A technique using a lignocellulosic biomass called the second generation biomass that is cheaper and has a less problem in supply of raw materials is actively being developed to overcome the above-described problems in the long term. As representative lignocellulosic biomass plants, poplars and eucalyptuses are used, and woods of these trees have attracted attention as an important material for producing various products such as papers, pulps, or fibers, and bioethanol. Due to such economic importance, studies for increasing the quantity and quality of woods are progressing, and thus a study for a synthesis pathway and a mechanism of controlling synthesis of woods is very important. A wood layer of a tree is composed of a secondary xylem, which is usually composed of a vessel element and fibers. In the angiosperm such as a poplar or eucalyptus, wood, that is, the secondary xylem, is formed in a vascular cambium, and completed through formation of a secondary cell wall and development of programmed cell death. Recently, the poplar is a model tree for study, and becomes an important plant for study as a tree for a biomass. Particularly, construction of various molecular biological databases and system establishment including decoding of gene sequence of the poplar, EST, microarray data accumulation, development of a molecular marker, and establishment of transforming technology become important bases for activating studies on poplars.
To develop the tree for a biomass, quantitative and qualitative increases in wood, that is, a xylem should be carried out. To this end, based on the understanding of a molecular mechanism of xylem development, it is essential to study the formation of a secondary cell wall and a process of the programmed cell death. The study on the formation of the secondary cell wall is very actively progressing, but relatively, the study on the programmed cell death is insignificant. Phenomenologically, the process of the programmed cell death sequentially includes vacuole collapse, secretion and activation of various lyases, and degradation of a cytoplasm, and thus formation of a xylem is observed. However, specific molecular mechanisms or critical regulatory factors are not mostly known. In the laboratory, autophagy is considered to be an important process to control the programmed cell death, and for the past several years, studies on the autophagy and the programmed cell death mechanism have been performed. Meanwhile, in the year of 2000, according to Fukuda Laboratory, probability of relating autography during the programmed cell death to differentiation of a vessel element has been disclosed, but there was no subsequent study thereon. To define a mechanism of xylem development derived from a vascular cambium, particularly, in addition to a molecular-level study on vessel element differentiation, during the vessel element differentiation, studies on the function of the autophagy and interrelation between the programmed cell death and the autophagy should be essentially performed. Accordingly, as the xylem development is stimulated and the qualitative and quantitative increases of wood layers are induced, development of lignocelluloses biomass plants ultimately useful in the industry can be successfully carried out.
To establish a “sugar platform” including various liquid fuels such as ethanol and butanol or value-added products in various ranges including a monomer of a raw polymer such as polylactic acid produced from fermentative sugar such as glucose, lignocellulose-derived biorefineries such as crude oil refineries now receive attention as a promising industry [Kamm, B., Kamm, M. 2004. Principles of biorefineries. Appl Microbiol Biotechnol, 64(2), 191 137-45; Lynd, L. R., Wyman, C. E., Gerngross, T. U. 1999. Biocommodity Engineering. Biotechnol 198 Prog, 15(5), 777-93]. Development of materials offering high cost efficiency is one of the main challenges for industrialization, and various studies relating to the development of an exclusive lignocellulose biomass are still progressing in the world including the U.S. and Europe. Due to a high growth rate and short-term circulation, the poplar is a representative biomass product in the art [Sannigrahi, P., Tuskan, G. A., Ragauskas, A. J. 2010. Poplar as a feedstock for biofuels: a 200 review of compositional characteristics. Biofuels Bioprod Bioref, 4, 209-226]. In addition, such a perennial tree is enriched with the main components for sugar turnover (conversion), which are cellulose and hemicelluloses, and does not need an input of large amounts of chemical products, and thus smaller investment in cultivation is needed [Baucher, M., Halpin, C., Petit-Conil, M., Boerjan, W. 2003. Lignin: genetic engineering and 166 impact on pulping. Crit Rev Biochem Mol Biol, 38(4), 305-50]. Until now, since the conversion into a lignocellulose is dependent on a specific gravity of wood and contents of lignin and cellulose, some studies relating to genetic engineering modifications have been performed. Details relating to genetic engineering improvement of poplars can be confirmed from the following literatures [Chen, F., Dixon, R. A. 2007. Lignin modification improves fermentable sugar yields for 168 biofuel production. Nat Biotechnol, 25(7), 759-61; Dinus, R. J. 2001a. Genetic improvement of poplar feedstock quality for ethanol production. 170 Appl Biochem Biotechnol, 91-93, 23-34; Sannigrahi, P., Tuskan, G. A., Ragauskas, A. J. 2010. Poplar as a feedstock for biofuels: a 200 review of compositional characteristics. Biofuels Bioprod Bioref, 4, 209-226]. In fact, the improvement of poplars through genetic modifications caused by 50% reduction of lignin was reported in 1999 [Service, R. F. 2007. Cellulosic ethanol. Biofuel researchers prepare to reap a new harvest. 204 Science, 315(5818), 1488-91]. However, until now, there is no study practically showing a systematic approach for utilizing genetically-modified plants including poplars to produce sugar.
The present inventors perform a study on differentiation of a vessel element as a representative cell layer of a xylem. Particularly, a small GTP-binding protein, RabG3b, which is found by analysis of secretive protein of Arabidopsis thaliana, is concerned, involved in the autophagy and vessel element differentiation, and probability of involving the autophagy in the vessel element differentiation is considered. Through various molecular-biological and cellular studies, it is known that the RabG3b is involved in vessel element differentiation in plants, that is, the xylem development, which is a mechanism performed by a function of the RabG3b to control the autophagy. Moreover, it is defined that the autophagy is an important process in the vessel element differentiation. When RabG3bCA is overexpressed in Arabidopsis thaliana, it is seen that both of length growth and thickness growth are increased compared to those of a wild-type plant, and it is confirmed that such growths are stimulated by the xylem development [Korean Patent Application No. 2010-0085609].